A band filter is a device that is able to separate an incoming optical spectrum into bands (i.e., when used as a demultiplexer) or combine separate optical bands into an output optical spectrum (i.e., when used as a multiplexer), ideally with sharp passband corners. Each band includes one or more optical channels from a multiplexed signal having a plurality of separate optical channels. Band filters are useful in both dense (D) and coarse (C) wavelength-division multiplexed (WDM) systems. In DWDM systems, band filters are typically used for de/multiplexing with a high spectral efficiency or for permitting the use of narrowband optical amplifiers, dispersion compensators, add-drop filters, etc. In CWDM systems, band filters are typically used for de/multiplexing without transmitter temperature control.
In current optical systems, a substantial portion of the implemented band filters are based on thin-film interference filters especially in CWDM applications. Thin-film interference filters typically have low loss over wide passbands. Thin-film interference filters also exhibit significant chromatic dispersion; they require more than ten cavities to achieve sharp passband corners which increases the filter size and complexity appreciably; and they typically require a cascade of elements to demultiplex more than two bands. As these filters are assembled into the final band filter package, they require extensive characterization and matching, tedious hand assembly, and expensive hermetic packages making mass production on any scale expensive.
Band filters based on bulk gratings have also been proposed. This type of band filter can be dispersion-free and can de/multiplex more than two bands without a cascade of elements. Bulk grating band filters also exhibit large form factors and, like thin-film filters, require tedious hand assembly and expensive hermetic packages. In addition, these filters exhibit more loss than their thin-film counterparts.
Band filters based on planar lightwave circuits (PLCs) have also been proposed. In contrast to the thin-film filter technology and the bulk grating filter technology described above, silica waveguide PLCs do not require hermetic packaging or internal optical alignment. PLC band filters are capable of being dispersion-free, and they exhibit sharp passband corners. However, previously proposed band filters based on PLC technology utilized cascaded Mach-Zender Interferometers (MZI), which are exceedingly large in form factor, are limited to de/multiplex only a few flat bands, and usually lack sharply defined passband corners. For CWDM applications in particular, the limitation on the number of bands arises because these filters are designed with a constant frequency difference between de/multiplexer channels rather than a constant wavelength difference as dictated by the CWDM scheme. One example of a four channel MZI-based CWDM filter is shown in an article by Y. Inoue et al. entitled “Low-crosstalk 4-channel coarse WDM filter using silica-based planar-lightwave-circuit,” Optical Fiber Conference Digest, Paper TuK6, pp. 75–76, Mar. 19, 2002.